Add CMutableTransaction and make CTransaction immutable.

In addition, introduce a cached hash inside CTransaction, to prevent
recalculating it over and over again.

src/qt/coincontroldialog.cpp

@@ -440,7 +440,7 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
     // nPayAmount
     qint64 nPayAmount = 0;
     bool fDust = false;
-    CTransaction txDummy;
+    CMutableTransaction txDummy;
     foreach(const qint64 &amount, CoinControlDialog::payAmounts)
     {
         nPayAmount += amount;